DIGITAL EDITION

In Pursuit of the Perfect Stencil

(October 2007) posted on Tue Oct 09, 2007

Excellence in stencilmaking yields screens that stand up to production wear and produce detailed, high-quality graphics. Find out how to regulate the instabilities in the process that can lead to less than desirable results.

Use an exposure calculator or 21-step grayscale to determine optimum exposure. An exposure calculator usually consists of a repeating piece of artwork overlaid with a series of increasingly darker gray neutral density filters. For example, one test exposure can simulate five different exposure times.

Examination of the developed and dried stencil reveals rectangles where the strong yellow color from residual unexposed diazo alters the color of the stencil. The trick is to pick the exposure factor for the rectangle that just becomes indistinguishable from the background—this corresponds to the optimum exposure time. An exposure time long enough to give seven solid steps on a developed stencil is generally very close to the optimum with a 21-step grayscale. Because photopolymers do not change color on exposure, the 21-step grayscale method is a more reliable method of determining optimum cure than an exposure calculator—although you can use the calculator to determine the level of resolution that you can achieve at different exposure times.

The use of direct emulsion makes it impossible to expose a collection of different mesh counts at once and en-sure that the correct exposure time is given. Longer exposure time is required for thicker coatings, and the coarser the mesh, the thicker the layer of emulsion that has to be cured.

Screen drying is another important variable that should not be overlooked as a cause of possible problems. Both capillary film and direct emulsions require very thorough drying prior to exposure, because any residual moisture present in the coating will react preferentially with the photosensitive resins that are supposed to harden the stencil. When you expose a damp screen, you end up with a stencil that exhibits the symptoms of having been underexposed—except that no improvement is ever seen when you increase exposure time.